Do not update CMAKE_REQUIRED_FLAGS to match CMAKE_CXX_FLAGS.

- This was a hangover from the pre-Ceres 2.0 era when we needed to
  ensure that -std=c++11 was passed to check_cxx_source_compiles() in
  FindUnorderedMap/FindSharedPtr if the user had enabled CXX11.
- Unfortunately, the same variable (CMAKE_REQUIRED_FLAGS) is used for
  both check_cxx_source_compiles() and check_c_source_compiles(), and
  thus updating it to include -std=c++11 would thus break any checks
  for C, which are made internally in the version of FindThreads.cmake
  shipped with CMake which could cause build breakage as identified
  in issue #355.

Change-Id: I36be1a21db14e3839eaf955a80bd952ef40fa269